1. Field of the Invention
The present invention relates to a method for recrystallizing fine spherical cyclotrimethylenetrinitramine (Research Department Explosive, hereinafter, referred to as “RDX”) particles, and more particularly, to a method for recrystallizing fine spherical RDX particles using a dimethylether compressed gas.
2. Background of the Invention
In case of high explosive powders and propellants, about 85 to 95 weight percent of total composition is used with a powder form, thus requiring an intermediate process in which chemical particles are controlled so as to have properties suitable for an end product from synthesized explosive chemicals prior to the final recrystallization process. The properties of those explosive chemicals may be determined by the size, distribution and shape of particles, which may be controlled to have properties needed for each application field by changing the recrystallization method or process condition.
The fine structure of explosive powder particles greatly affects on a combustion or explosive behavior thereof wherein heat or mass transfer rate governs the energy generation speed, and its transfer rate may be primarily determined by the size of particles or uniformity of the end product. Accordingly, the rate-limiting step governing an overall reaction speed may be made from a heat or mass transfer process to a chemical reaction step by recrystallizing particles constituting the end product in a very small size. Due to this reason, a lot of studies on recrystallizing fine explosive particles have been carried out, and it has been reported that they have a much higher combustion speed and a low shock sensitivity, and a temperature of generating the maximum energy is also low.
However, powder particles used to recrystallize high explosive powders and propellants are weak against heat and shock and thus a wet process should be used therefor, but it is known there is a limit in recrystallizing particles having a size of less than one micrometer in the related art using a milling method, a recrystallization from solution method, an emulsion crystallization method, a spray crystallization method, an ultrasonic method, or the like. In particular, in case of recrystallizing small particles having a nano size, it is known difficult to control the size or distribution of particles using the grinding or recrystallization from solution method which is primarily used in the existing industries. On the other hand, in case of the spray crystallization/drying process, it is known that even particles with a size of less than 5 μm can be recrystallized, but the process is not suitable to recrystallize powder particles since a high-temperature evaporation process for removing solvent is required. Furthermore, it is difficult to prevent a particle crystalline phase from being contaminated with solvent in case of a wet crystallization process, and it has a fundamental disadvantage in applying to the recrystallization of powder particles weak to shock in case of a jet milling process.
Supercritical fluid is defined as “a fluid at a temperature and pressure above its critical point”, and has distinct characteristics not shown in the existing solvents. The properties of a solvent may be determined according to interactions between molecules determined by the kind of molecules and distance between molecules. Accordingly, the distance between molecules is not almost changed because liquid solvent is incompressible and thus it may be difficult to expect a great change of the properties as a single solvent. Supercritical fluid can continuously change density from a sparse state close to ideal gas to a high-density state close to liquid density, thereby regulating a solvation and molecular clustering state as well as equilibrium properties (solubility, entrainer effect), transfer properties (viscosity, diffusion coefficient, thermal conductivity) in a fluid. Supercritical fluid process is a process of selectively separating and recrystallizing a high pure and higher value-added product at a high mass transfer rate using thermodynamic characteristics (high solubility, selectivity, compressibility, and voluntary separation due to decompression) and flow properties (low viscosity, standard tension, high diffusion coefficient) and the like, and maximizing an energy use efficiency during the extraction, separation and drying process with the reuse of extraction solvent and the removal of the remaining solvent in the product.
For the process of recrystallizing particles using a supercritical fluid, there are used a rapid expansion of supercritical solutions (RESS), a supercritical antisolvent precipitation (SAS), a particle generation from gas-saturated solutions (PGSS), and the like. In particular, among the above processes, the RESS process may be advantageous if a solute can be dissolved in a supercritical fluid, but the SAS process may be advantageous if a solute cannot be dissolved. However, the RESS process using supercritical carbon dioxide may be applicable to only a specific supercritical material and the solubility of the majority of materials of interest is low, thus having a disadvantage that a great amount of supercritical carbon dioxide should be consumed even when recrystallizing a small quantity of powder particles. Due to this, the RESS process using supercritical carbon dioxide has not been widely used because of having a low efficiency in recrystallizing fine powder particles.
Furthermore, the size of the existing RDX particles recrystallized using the recrystallization from solution process is large and the particle distribution is not uniform, thus having a limit in enhancing explosive performance. Accordingly, it may be required to enhance powder performance by miniaturizing the size of particles. However, the sensitivity of powder tends to be increased as reducing the size of particles, and thus studies for recrystallizing fine spherical RDX particles having insensibility while enhancing the performance of powder are needed. However, technologies for enhancing RDX particles recrystallization and productivity required in the above using a supercritical process have not been reported until now.